MIMO (multiple-input multiple-output) transmission technology is becoming essential as a next-generation wireless communication technology due to its spatial multiplexing and diversity effects. For its mounting, however, a system that mounts a plurality of transmitting/receiving apparatuses and antennas with equivalent characteristics merely in parallel is a mainstream. As a result, there are two problems, i.e., increases in the numbers of input terminals, chip size and power consumption of RFIC (radio frequency integrated circuit), and insufficient isolation between the antennas.
Methods shown in Patent Literatures 1 and 2, and Non-Patent Literatures 1 and 2, for example, have been known as means for solving the first problem of increases in the numbers of input terminals, chip size and power consumption of RFIC caused by the parallel mounting. Thus conventional methods are used by switching a single RF (radio frequency) front end by the time for a plurality of antennas.
FIG. 1 is a block diagram showing the configuration of a conventional MIMO-enabled wireless receiving apparatus described in Patent Literature 1, hereinafter the explanation is focused to a case of the number N of branches being two for simplification.
The wireless receiving apparatus shown in FIG. 1 comprises an antenna array consisting of two receiving antennas 2101 and 2102 to receive RF signals, 2-to-1 multiplexer 2020, disposed at the output side of two receiving antennas 2101 and 2102, for multiplexing two channel signals into one output, down-converter 2140, disposed at the output side of 2-to-1 multiplexer 2020, for down-converting the RF signals to baseband, two 1-to-2 analog demultiplexers 2061 and 2062, disposed at the output side of the down converter, for demultiplexing each of two multiplexed received signals to an in-phase component and a quadrature component. respectively, and four low-pass filters, disposed at the output sides of the two 1-to-2 analog demultiplexers, for filtering and reconfiguring baseband signals with two in-phase components or two quadrature components.
In the wireless receiving apparatus, two signals received through two receiving antennas 2101 and 2102 are multiplexed into one output on a time division basis by 2-to-1 multiplexer 2020. The multiplexed signal is then down-converted to the baseband signal by down-converter 2140, and the in-phase component and the quadrature component are output. Each of the two received signals multiplexed and down-converted are demultiplexed to two in-phase components of the received signal and two quadrature components of the received signals by two 1-to-2 analog demultiplexers 2061 and 2062, respectively.
In such a manner, down-converter 2140 down-converts the multiplexed signal of the received signals in the wireless receiving apparatus. Thus, the wireless receiving apparatus has a simplified RF front end configuration that can reduce power consumption compared to a mainstream system of MIMO or SISO in which individual receivers mounted in parallel down-convert the multiplexed signal.
Methods described in, for example, Patent Literatures 3 to 10 are known as measures for solving the second problem of the insufficient isolation between the antennas. That is, a conventional method provided with passive antennas and variable reactance elements in proximity to feed antenna elements performs optimal control of an antenna radiation pattern. FIG. 2 is a block diagram showing a configuration of a conventional MIMO receiving antenna device described in the Patent Literature 3.
In MIMO receiving antenna device 3001 shown in FIG. 2, antenna elements at both sides among antenna elements arranged. in a row arc feed antenna elements 3011 and 3012. The other elements disposed between feed antenna elements 3011 and 3012 at the both sides are passive antenna elements 3021 and 3022. Passive antenna elements 3021 and 3022 are terminated by reactance elements 3031 and 3032. This method sets reactance values of reactance elements 3031 and 3032 to a value that maximizes the expectation value of communication capacity in wireless communication. This method can reduce antenna coupling between a plurality of feed antenna elements and can increase SNR. (Signal to Noise Ratio). The method can also reduce the spatial correlation between the feed antenna elements to increase the communication capacity.